A read channel is the circuitry used in storage systems for reading data from a storage medium such as a rotating magnetic disk. A magnetic read head transduces magnetic transitions recorded on the storage medium into pulses in an analog read signal, and these pulses are detected and decoded by the read channel circuitry into a digital sequence. Decoding the pulses into a digital sequence can be performed by a simple pulse detector read channel or, as in more recent designs, using a partial response maximum likelihood (PRML) read channel. See, for example, Roy D. Cideciyan, Francois Dolivo, Walter Hirt, and Wolfgang Schott, "A PRML System for Digital Magnetic Recording", IEEE Journal on Selected Areas in Communications, Vol. 10 No. 1, Jan. 1992, pp.38-56.
The more complex PRML read channels are comprised of many different components such as an analog pulse equalizing filter, an analog to digital converter, a digital equalizing filter, a pole tip filter, a timing recovery circuit, an automatic gain control circuit, and a maximum likelihood (ML) sequence detector. These components can be integrated into a single read channel integrated circuit (IC) having mixed analog/digital circuitry. PRML read channels are preferred over the simpler pulse detection schemes because they decrease the necessary bandwidth allowing more data to be stored on the storage medium. However due to their complexity, the performance and accuracy of a PRML channel depends on the performance and accuracy of each component therein.
In order to operate optimally, the components of the read channel must be tuned to the particular operating characteristics of the storage system. For instance, the read channel can be tuned to operate according to the characteristics of a particular magnetic read head, a particular height the head operates above the storage medium, or a particular speed at which the magnetic storage medium is moving, as in a magnetic disk spinning. Also, the characteristics of the storage medium vary between each disk drive because there are slight but significant differences in composition and in manufacturing. These characteristics of the storage medium may degrade over time as well. Therefore, the read channel should be tuned to operate according to the characteristics of a particular drive as well as the characteristics of the magnetic storage medium. A further consideration is that in magnetic disk storage systems, the characteristics of the magnetic disk change as the track radii change. For these systems, the disk is partitioned into several zones having a varying data density per track and the read channel is tuned to operate in each zone.
The prior art techniques for calibrating the read channel can be separated into two general groups, adaptive and non-adaptive systems. Adaptive calibration systems, such as those disclosed in U.S. Pat. No. 5,132,988 and in U.S. Pat. No. 5,150,379, utilize characteristics of the data as it is being read to "adapt" the parameters of the read channel. Thus the components of the read channel are continuously calibrated to operate at an optimum level. The problem with adaptive techniques is that they involve more complex and expensive signal processing circuitry, and they can exhibit systematic errors in converging to a proper value due to instabilities and noise in the system.
There are non-adaptive calibration techniques that are not as complex as adaptive systems and are not prone to non-convergence problems. For instance, in U.S. Pat. No. 5,121,260 a read channel optimization system is disclosed where the characteristics of the storage system and storage medium are measured when the storage system is manufactured. The optimization comprises writing and reading test data to and from the storage medium and measuring the system characteristics as the data is processed by the read channel. Well known instrumentation for measuring the characteristics are used, such as oscilloscopes and spectrum analyzers, and the measurement data for each zone is stored at a specific location on the storage medium. During operation, the characteristic data is read from the storage medium and used to calibrate the read channel.
The problem with this technique is that the instrumentation used to measure the characteristic data is sophisticated and expensive. Further, it is difficult to automate the system of measuring and storing the data and the number of measurement instruments is limited. Yet another problem with this technique is that it does not compensate for changes over time because the calibration is performed only during manufacturing. Finally, if the storage system is repaired by replacing a component or changing the storage medium, the system must be re-calibrated using the sophisticated measuring equipment.
Another deficiency with prior art methods for calibrating a read channel is the inability to tune the components in a particular sequence to achieve optimum performance. For instance, the parameters of one component may affect the operation of another component, e.g. the parameters of the analog equalizing filter affect the automatic gain control circuit and timing recovery circuit. Thus the performance of these circuits is affected by the sequence in which they are calibrated.
Yet another deficiency with prior art calibration techniques is the inability to compensate for asymmetrical pulses in a PRML read channel. Asymmetrical pulses cause errors in the gain control and timing recovery circuits due to incorrect "a", "b", "1", and "c" target detector levels.
A further desired feature not provided by prior art optimization techniques is the ability to verify the performance of a read channel by testing the bit error rate after a calibration and to re-calibrate the read channel successively until an optimum bit error rate is achieved.
Thus, it is a general object of the invention to provide an easy and inexpensive method to automatically calibrate the components of a read channel IC to operate according to the characteristics of a particular storage system that does not exhibit the non-convergence problems associated with adaptive techniques, nor requires sophisticated measuring equipment. Another object is to provide an automated calibration method that can be performed throughout the life of the storage system to compensate for changes occurring over time. Yet another object is to calibrate the components of the read channel in a specific sequence or in combination to achieve optimum performance. Still another object is to generate a bit error rate of the system after calibration and to re-calibrate the components until a desired minimum bit error rate is achieved.